This invention relates to image sensors and, more particularly, to improvements in charge storage type image sensors.
Recently, image sensors of close contact type, which lead originals in the same scale, are adopted for original reading sections of facsimile and like systems in place of CCD type image sensors requiring contracting optical systems. The close contact type image sensor comprises a number of photo-diodes, which are formed in a unidimensional arrangement on an insulating substrate of glass or the like and comprise a thin semiconductor film of amorphous silicon a-Si. Usually, this type of image sensor adopts a matrix drive system for reading out signals from the photo-diodes sequentially in block-by-block reading.
FIG. 12 shows a prior art matrix drive system image sensor. This image sensor comprises in an array m by n diode pairs each including a photo- and a switching blocking diode PD and BD with the cathode electrodes thereof connected in series. The blocking diodes BD have their anode electrodes connected commonly in the individual blocks B.sub.1,B.sub.2, . . . , Bn by respective block leads A.sub.1,A.sub.2, . . . , An. The photo-diodes PD have their like electrodes in the individual blocks B.sub.1,B.sub.2, . . . , Bn connected commonly by respective channel leads C.sub.1,C.sub.2, . . . , Cm.
This image sensor is based on a commonly termed charge storage system for its operation. As shown in the time chart of FIG. 13, drive pulses Vp.sub.1, Vp.sub.2, . . . , Vpn are applied sequentially to the respective blocks B.sub.1,B.sub.2, . . . , Bn. In response to the application of these drive pulses, photo-currents Io.sub.1, Io.sub.2, . . . , Iom corresponding to stored light signals are caused to flow into the photo-diodes PD in the individual blocks B.sub.1,B.sub.2, . . . , Bn through the channel leads C.sub.1,C.sub.2, . . . , Cm to be processed by integration, amplification and so forth in a signal processing circuit (not shown), thus obtaining an image signal.
This operation will be described in greater detail in connection with one diode pair.
As shown in FIG. 14, the anode electrode of the photo-diode PD in the diode pair is connected through one of the channel leads C.sub.1,C.sub.2, . . . , Cm to an input terminal of the signal processing circuit. Usually, it is thus held at zero potential. When a drive pulse Vp is applied such as to bias the blocking diode BD forwardly and bias the photo-diode PD reversely as shown in FIG. 15, the capacitance of the photo-diode PD starts to be charged, causing the potential Vj at the juncture between the photo- and blocking diodes PD and BD (hereinafter referred to as mid point potential) to increase toward the drive pulse voltage Vp.
When the application of the drive pulse Vp is ended, the blocking diode BD is in a reversely biased state, and the capacitance of the photo-diode PD is gradually discharged by photo-current generated according to incident light intensity and the mid point potential is gradually decreasing. Denoting the interval of the drive pulses Vp (hereinafter referred to as storage time) by T and the photo-current generated according to the incident light intensity by Ip, the charge .DELTA. Q which is discharged during the storage time T is given as EQU .DELTA.Q=Ip.multidot.T
Further, denoting the capacitance of the photo-diode PD by C, the change a .DELTA. in the mid point voltage Vj is given as EQU .DELTA.V=.DELTA.Q/C=Ip.multidot.T/C
When the next drive pulse Vp is applied, the capacitance of the photo-diode PD starts to be charged again, causing the mid point potential Vj to increase toward the voltage Vp. At this time, a charging current is caused to flow through the photo-diode PD to make up for the charged .DELTA. Q. Through time integration of this charging current, an electric signal corresponding to the photo-signal stored in the photo-diode PD during the storage time T can be obtained as an image signal.
The changes in the mid point potential Vj include level change currents generated due to the capacitances of the photo- and blocking diodes PD and BD at the time of the rising and falling of the drive pulses Vp, as shown in FIG. 15. These level change currents are canceled as much as possible by making the falling of a drive pulse Vp for each of the blocks B.sub.1,B.sub.2, . . . , Bn and the rising of the next drive pulse Vp for the next block coincide in timing.
While this image sensor is comparatively simple in construction, it has the following drawbacks.
In the first place, it is difficult to obtain high speed operation. For allowing high speed operation, it is necessary to secure sufficient magnitude of the forward current in the blocking diode BD, and hence the forward voltage Vf applied thereto, so as to permit quick charge of the capacitance of the photo-diode PD. The forward voltage Vf applied to the blocking diode BD is given as EQU Vf=Vp-Vj
Meanwhile, for obtaining substantially complete charging of the capacitance of the photo-diode PD which is made to discharge by the photo-current Ip, it is necessary to wait for the mid point potential Vj to reach substantial Vp. However, the forward voltage Vf is reduced substantially to zero by the time when the application of the drive pulse Vp is ended. In addition, when the illumination intensity of original read out is low, at which time the incident light intensity is low, the voltage change.DELTA. V is small. In this case, the forward voltage Vf is already low in an initial stage of application of the drive pulse Vp. This means that prolonged operation is dictated under low illumination intensity.
A second drawback is that residual image is produced in the vertical direction. As noted above, for obtaining substantially complete charging of the capacitance of the photo-diode PD discharged by the photo-current Ip, it is necessary to wait for the mid point potential Vj to reach substantial Vp. This requires a considerably long duration of the drive pulse Vp, which is infeasible. In practice, the charging is discontinued at a certain timing. Therefore, in case of a change in the original state from white to black as shown in FIG. 16, in spite of the reading of black original a charging current cause the mid point potential Vj to be increased by .DELTA. Vres. In consequence, a residual image is generated in the vertical direction.
The above drawbacks are fatal problems inherent to the charge storage system, and various measures have been proposed. However, such measures are mostly complicated in structure, and those feasible for use are scanty.
The inventors, accordingly, conducted extensive researches and investigations in order to solve all the above problems and provide an image sensor, which operates at high speed, is free from residual image and is inexpensive. The invention is completed as a result of these researches and investigations.